The early detection of breast tumors at a stage when it is most treatable represents the most important issue in the clinical management of breast cancer. Although screening mammography is widely accepted as the best procedure for this purpose, there are a number conditions (i.e., women with dense breast tissue, breast implants, or scar tissue for an earlier biopsy study) that interfere with the identification of abnormalities that may be caused by breast tumors. In addition, another major problem in the clinical management of breast cancer is the identification of an appropriate treatment strategy once the disease has been correctly diagnosed. Therefore, a noninvasive imaging procedure that can detect breast tumors in patients that are problematic for mammography, and provide insight into identifying an appropriate strategy for treating this disease, would be an important tool in the clinical management of breast cancer. The goal of the research described in this application is to develop sigma-2 (sigma2) receptor-based radiotracers for imaging breast tumors with the noninvasive imaging technique PET. The choice of this receptor as a target for imaging is based on published reports that human breast tumors possess a high density of sigma2 receptors versus surrounding normal tissue. Therefore, a sigma2 receptor-based imaging agent should be useful in detecting breast tumors is patients that are problematic for mammography. A second reason for focusing on this receptor is our published data demonstrating that sigma2 receptors are expressed in approximately 10-fold higher density in proliferative mouse mammary adenocarcinoma cells versus the nonproliferative or quiescent cell population under both in vitro (cell culture) and in vivo (tumor xenografts) conditions. Therefore, a sigma2 receptor PET radiotracer has the potential to provide information regarding the proliferative status of breast cancer. An in vivo imaging procedure that can provide information about the proliferative status of primary breast tumors would represent a significant improvement over current methods used in making this assessment. Our preliminary data also indicate that sigma2-selective radiotracers are predicted to have a better tumor: background ratio than other agents, such as FDG and the DNA precursors, currently used to assess proliferation in PET oncology studies. Consequently, a major goal of this R21/R33 grant application is to conduct the necessary in vivo studies needed to validate this novel, receptor-based imaging approach for determining the proliferative status of breast tumors. A series of studies are also proposed to compare our imaging procedure with that of [18F]FLT, a substrate of thymidine kinase 1 which was recently introduced as a potential proliferation-based radiotracer. We propose to compare our receptor-based approach to the [18F]FLT method, and compare the results of these studies to proliferative status of the tumor determined by flow cytometry of the BUdR labeled DNA. Finally, a series of microPET imaging studies (using the Concorde Microsystems microPET R4 and Focus PET scanners) will be conducted in order to determine if the sigma2 receptor imaging approach can be used to monitor a positive response to antiestrogen and Herceptin therapy in estrogen receptor (+) and Her2/neu receptor (+) human breast tumor xenografts.